No question that Vito saw politics as a more legitimate form of crime. The power and influence that Mike would have wielded was the brass ring for the Corleones. I'm only speculating here, but maybe on a deeper level, Vito wanted some redemption (in his own mind) of his legacy. One son is a hot headed, psychotic leg breaker,and the other a dim-witted weakling. How nice to be able to reflect back on his life and take some solace in the fact that at least one son was the one "holding the strings".